BREAKING CHANGE:
Previously, `Directive` was the abstract base class of several directives.
Now, `Directive` is the former `Decorator`, and `Component` inherits from it.
BREAKING_CHANGE:
- The special type of `Viewport` directives is removed
in favor of a more general `Decorator` directive
- `ViewContainerRef` now no more has a default `ProtoViewRef`
but requires an explicit one when creating views.
Closes#1536